Photosensitive devices, such as image sensors and PV cells convert light intensities into an electrical signal. An example of an image sensor is a contact image sensor (CIS) that will convert light energy into a voltage. The photodiode is responsible for such photovoltage conversion and this voltage may later be converted into digital data by complementary metal oxide semiconductor (CMOS) circuitry linked to the image sensor.
FIG. 1 is a cross-sectional view of an exemplary photosensitive device, specifically an image sensor 150. The sensor 150 is located on a workpiece, such as a silicon substrate. The photo-diode region 151 includes the P-doped region 152 on the N-doped well 153 contained within a lightly P-doped bulk region 154. In other embodiments, regions 152 and 154 may be N-doped, while region 153 is P-doped. To increase the density of photo-diodes on the surface of a substrate, it may be necessary to isolate them from one another, as leakage may occur between them. One such isolation technique is to incorporate trenches beside or adjacent to each photo-diode, which extend below the photo-diode 151. In FIG. 1, a shallow trench isolation (STI) 155 is disposed next to the photo-diode region 151 in this particular embodiment. A P-well 156 surrounds the STI 155. The photo diode region 151, or the P-N-P doped region, is the image sensor. Another P-well and N-doped region may be disposed adjacent the photo-diode region 151 opposite the STI 155 in one particular embodiment. FIG. 2 is another cross-sectional view of an exemplary image sensor. This figure also shows the mechanisms used to read the voltage stored by the photo-diode, and to reset that voltage.
A second type of image sensor is the back side illuminated (BSI) image sensor. As the name suggests, light enters these devices from the back side (rather than the front side). Like the CIS described above, the BSI sensor utilizes the p-n junction to achieve charge separation.
Another type of image sensor is the charge-coupled device (CCD) image sensor. When light strikes the CCD image sensor, it is held as an electrical charge in the image sensor. The charges are converted to a voltage as these charges are read from the chip containing the CCD image sensor. This voltage may later be converted into digital data by circuitry linked to the CCD image sensor.
FIG. 3 is a cross-sectional view of an exemplary photovoltaic (PV) cell. Other embodiments or designs are possible and the embodiments of the process described herein are not solely limited to the PV cell 120 illustrated in FIG. 3. PV cell 120 includes contacts 121 and backside contact 125. Underneath the dielectric 122 is the emitter 123 and base 124 that make up the P-N junction in the PV cell 120. The emitter 123 and base 124 may be either P-type or N-type depending on the PV cell 120 design. In some instances, this dielectric 122 may be a dielectric passivation layer or an antireflective coating.
As light strikes the PV cell, the photons with sufficient energy (above the bandgap of the semiconductor) are able to promote an electron within the semiconductor material's valence band to the conduction band. Associated with this free electron is a corresponding positively charged hole in the valence band. In order to generate a photocurrent that can drive an external load, these electron hole (e-h) pairs need to be separated. This is done through the built-in electric field at the p-n junction. Thus any e-h pairs that are generated in the depletion region of the p-n junction get separated, as are any other minority carriers that diffuse to the depletion region of the device. Since a majority of the incident photons are absorbed in near surface regions of the device, the minority carriers generated in the emitter need to diffuse across the depth of the emitter to reach the depletion region and get swept across to the other side. Thus to maximize the collection of photo-generated current and minimize the chances of carrier recombination in the emitter, it is preferable to have the emitter region 123 be very shallow.
Some photons pass through the emitter region 123 and enter the base 124. These photons can then excite electrons within the base 124, which are free to move into the emitter region 123, while the associated holes remain in the base 124. As a result of the charge separation caused by the presence of this p-n junction, the extra carriers (electrons and holes) generated by the photons can then be used to drive an external load to complete the circuit.
By externally connecting the emitter region 123 to the base 124 through an external load, it is possible to conduct current and therefore provide power. To achieve this, contacts 121, 125, typically metallic, are placed on the outer surface of the emitter region and the base. Since the base does not receive the photons directly, typically its backside contact 125 is placed along the entire outer surface. In contrast, the outer surface of the emitter region 123 receives photons and therefore cannot be completely covered with contacts.
Performance degradation in both CMOS image sensors and CCD image sensors is driven in part due to parasitic current in the doped junctions that is known as “dark current.” Dark current is a parasitic electric current generated by the photo diode in an image sensor that originates due to inherent defectivity in the diode. Defects (such as unpassivated Si vacancies, Si interstitials, interstitial dopants, metal contamination, stacking faults, and dislocations) in the diode act as traps for minority carriers and when the diode is placed in reverse bias, these captured carriers are released. This reverse bias leakage current is referred to as the dark current. Thus, the charge generation rate of the dark current is related to the crystallographic defects in the image sensor, especially in the junction and at the surface of the image sensor. These dark currents degrade the signal-to-noise (S/N) ratio of the image sensor, which may be important to performance of the image sensor.
In an analogous fashion, dark current (or reverse saturation currents, as it is referred to in PV cells) in the P-N junction in a PV cell also is due to inherent defectivity in the junction. This interface degrades minority carrier lifetime in the substrate and leads to the degradation of the efficiency of the PV cell.
FIGS. 4A-4D are an embodiment of image sensor fabrication. In FIG. 4A, the trench 160 is etched. The corners 169 and walls of the trench 160 may be damaged during this etch. Etching will create silicon vacancy clusters (stacking fault nucleation sites) on the sides 170 of the trench 160. Etching also will create irregular asked areas with charge accumulation. In addition, the etch process damages the silicon at the etched surface, often creating dangling bonds. In FIG. 4B, an oxidation and/or nitridation step is performed, creating layer 161. In FIG. 4C, a high density plasma chemical vapor deposition (HDP CVD) step fills the trench 160 with material 162. The layer 161 and material 162 will exert stress and will grow any defects in the trench 160. The excess material 162 is removed using a chemical mechanical polish (CMP) step. FIG. 4D is a finished image sensor. A P-well 168, P-doped region 163, N-doped region 164, N-doped region 166, P-well 165, and transfer gate (TG) 167 are added to the image sensor. Compressive stress (represented by the arrows 600) may be exerted by the trench 160 on the P-doped region 163 and N-doped region 164.
Increasing stress, especially in the STI well, may increase dark current. Stress by HDP CVD may occur when an oxide is used to fill in the STI well 160. During the etching of the STI well 160 (i.e., before HDP CVD), the walls of the silicon are damaged. Defects will grow and proliferate during subsequent processing steps, such as oxide densification or chemical mechanical polishing (CMP). FIG. 5 is a transmission electron microscope (TEM) photographs illustrating dislocations at STI well corners.
The crystallographic defects that cause dark current may be due to multiple sources. First, the defects may be caused by residual damage in the crystal structure following doping, annealing, etching, or other integrated circuit (IC) processes. FIG. 11 shows several sources of dark current in an image sensor, such as the one shown in FIG. 1. The first source of dark current 901 is surface dark current, where imperfections at the interface between the surface and oxide top surface create defects. These defects trap minority carriers. The second cause of dark current 902 is depletion dark current, which is the generation of carriers in the depletion region, caused by interstitials, EOR defects and other substrate defects in the P-doped region. The third cause of dark currents 903 is diffusion of carriers from the doped region, and the fourth source 904 is the diffusion of carriers from the bulk region. These four sources of dark current are all caused by anomalies in the silicon crystal structure, typically created by annealing, implanting, etching or some other IC processing step.
Second, the defects may grow due to induced stress from material, structural, or image sensor layout irregularities. For example, trench isolation feature edges will nucleate defects because the trench corners and sidewalls are rough and defective. Filling the trenches using a HDP CVD will exert a stress on these defects and may proliferate the size of the defects. FIG. 11 shows the current 905 caused by feature related stress.
Dark currents in image sensors are good indicators of the quality of the junction or materials. Hence they are directly related to minority carrier lifetime. Minority carriers in a given doped area are the less abundant charge carrier, which can be either electrons or holes. Carrier lifetime is the average time needed for an excess minority carrier to recombine (i.e., for an electron to combine with a hole or passivate a defect). Light incident on an image sensor will create carriers that are collected and measured as a generated current. Measuring these dark currents in silicon in the presence of external incident light is difficult, as light induced generation of carriers will result in background noise. Isolating the silicon in an isolation system without the presence of light will result in the noise level being lowered and hence enable measuring of the dark current. Reverse biasing a diode in this isolation system allows the true characterization of any inherent defects in the silicon by measuring the leakage current. Existing defects will begin releasing trapped minority carriers, resulting in dark current. Generation lifetime of minority carriers where the capacitor operates in deep depletion is also related to dark currents. Dark current may degrade the S/N ratio of the image sensor and, consequently, may degrade the yield of the image sensor.
Previously, implantation of the STI well using BF3 has been performed at approximately 1E15 to 3E15 using plasma doping to reduce dark currents. Such a high dose of BF3, however, may cause parasitic currents in the image sensor. Other high dose implants also may cause damage or defects to the image sensor. Similarly, the doping of ions in the p and n regions in the photo diode may cause defects in the material, which lead to increased dark current.
Accordingly, there is a need for improved methods implanting a species in an image sensor or PV cell, and, more specifically, to implantation of a species in an image sensor or PV cell to improve dark currents and reduce defects.